1. Field of the Invention
The present invention generally relates to an image quality improvement treatment liquid, an image quality improvement processing method, an image forming method, and an image forming apparatus. More particularly, the present invention relates to a technique of improving image quality by preventing color bleeding and feathering occurring on a recording medium in a method or apparatus where an image in accordance with image information is formed on the recording medium such as a sheet using ink droplets from an inkjet.
2. Description of the Related Art
In an inkjet recording technique, ink is split into ink droplets by passing the ink through small (spray) nozzles and ejecting onto a recording medium like a sheet by using methods such as a pressure-on-demand method or a charge control method. Such a technique is preferably used in various image forming apparatuses such as printers, facsimile machines, and copiers. The inkjet recording techniques are expected to be further developed as the image recording methods for recoding images onto recording media because ink is directly ejected onto a recording medium, which makes it possible to reduce the size of the apparatus and manufacture the apparatus more easily as well when compared with a technique such as an electrophotographic recording technique which is an indirect printing type technique employed in an apparatus using a photosensitive body.
Next, an inkjet recording technique is described with reference to the accompanying figures. As shown in FIGS. 1A and 1B, when an ink droplet 63 including vehicle liquid 61 having water as its main component and color materials 62 including pigment and dye and dispersed in the vehicle liquid 61 as shown in FIG. 1A is ejected from an inkjet head and applied to a medium 64 such as non-coated paper like high-quality paper and coarse paper as shown in FIG. 1B, the vehicle liquid 61 and the color materials 62 penetrate along the direction of the pulp fibers (not shown) of the medium (sheet) 64. Because of the penetration along the direction of the pulp fibers on the surface of the sheet, the ink flows sideways to form a jagged shaped ink dot called “feathering”. Further, in forming a color image, after a first-color droplet layer is formed on a sheet, second-color droplets are ejected on the first-color droplet layer that is not yet dried on the sheet, causing the shape of the second-color droplet to be deformed and resulting in the ink flowing on the surface of the sheet, thereby generating bleeding of the ink called “color bleeding”. Further, most of the color materials 62 may penetrate inside the sheet, which may reduce the density of the image on the surface of the sheet and increase the density on the back side of the sheet, this phenomenon may be called “strike through of image”. Further, when an ink droplet is in contact with any other ink droplet on the surface of the sheet before penetrating inside the sheet, those ink droplets may be combined to form a dot which may become more than two times as large as normal dots called “beading” and appear granular. Namely, as described above, the ink droplets applied to the surface of a recording medium such as a sheet and a resin film may penetrate to the back side of the medium (the strike though), attach to the back side of another sheet (called “setoff”) when continuous printing is performed, and cause the feathering, the beading, the color bleeding, and “mottling” which is uneven density of the ink droplet on the recording medium.
To overcome the problems, according to Patent Documents 1 through 3, such feathering and color bleeding are prevented by applying an ink treatment liquid (hereinafter referred to as an “image quality improvement treatment liquid”) on a recording surface of the recording medium such as the high-quality paper or the coarse paper immediately before the ink droplets are ejected to the surface of the recording medium so that the applied ink droplets are fixed on the surface of the recording medium by the image quality improvement treatment liquid. Next, this prior-art technique is described in detail with reference to the accompanying drawings. As shown in FIG. 2A, when the high-quality paper or the coarse paper is used as the medium (sheet) 64, the image quality improvement treatment liquid 65 for fixing the color materials 62 of the ink droplets 63 is applied in advance on the surface of the medium (sheet) 64 to form an image quality improvement treatment liquid layer before the ink droplets 63 are applied (ejected) to the surface of the medium (sheet) so that a high-quality image is formed by the ink droplet 63 ejected onto the surface of the medium (sheet) 64. As shown in FIGS. 2B and 2C, when the ink droplet 63 is ejected onto the formed image quality improvement treatment liquid layer, the color materials 62 in the ink droplet 63 are agglutinated and fixed together so as not to penetrate along the directions of the pulp fibers of the medium (sheet) 64. As a result, the color materials 62 remain on the surface of the medium (sheet) 64 and on the other hand the vehicle liquid 61 penetrates into the medium (sheet) 64. This feature makes it possible to prevent the feathering, the color bleeding, the reduction of the density of an image, and the strike through of the image. Similarly, when a resin film is used as the medium (sheet) 64, the vehicle liquid 61 remains on the surface of the film but the color materials 62 can hardly be moved because the color materials 62 are likely to be agglutinated due to the image quality improvement treatment liquid 65, thereby enabling preventing the bleeding.
In order to make it possible to agglutinate the color materials 62 included in the ink droplet 63, it is necessary that the color materials 62 in the ink droplet 63 are negatively or positively charged by themselves. The dye itself is positively or negatively ionized in water. On the other hand, when a self-dispersion-type pigment is used as the pigment, the self-dispersion-type pigment itself is positively or negatively ionized in water. Further, when a pigment dispersion agent is used, the pigment dispersion agent is adsorbed onto the pigment and positively or negatively ionized in water. As a result, or equivalently, the pigment itself adsorbed by the pigment dispersion agent is positively or negatively ionized. Generally, the color materials are negatively charged and dispersed in the ink.
FIGS. 3A through 3C show a first principle of the operation of the image quality improvement treatment liquid. FIG. 3A shows the ink having water in which the color materials 62 are negatively ionized and dispersed. When the ink is in contact with the image quality improvement treatment liquid having acidity and including a large amount of protons (positive charges) as shown in FIG. 3B, the color materials 62 negatively ionized are electrostatically coupled with each other via the large amount of the protons in the image quality improvement treatment liquid so that the color materials 62 are agglutinated together as shown in FIG. 3C.
FIGS. 4A through 4C show a second principle of the operation of the image quality improvement treatment liquid. FIG. 4A shows the ink having water in which the color materials 62 are negatively ionized and dispersed. When the ink is in contact with the image quality improvement treatment liquid including cationic components indicating positive charges as shown in FIG. 4B, the color materials 62 negatively ionized are electrostatically coupled with each other via the cationic components in the image quality improvement treatment liquid so that the color materials 62 are agglutinated together as shown in FIG. 4C.
However, in order to apply the image quality improvement treatment liquid to the recording medium, a dedicated inkjet head for producing droplets of the image quality improvement treatment liquid has been used. Therefore, depending on a component included in the image quality improvement treatment liquid, the nozzle holes of the inkjet head may be clogged, which lacks reliability. Further, in order to successfully produce the ink droplets in the inkjet head, it may be necessary that a viscosity of the image quality improvement treatment liquid is as low as that of water, which limits the maximum viscosity of the image quality improvement treatment liquid. As a result of the limitation, there may be cases where, for example, even when there is an image quality improvement treatment liquid capable of effectively preventing ink bleeding but has a high viscosity, the image quality improvement treatment liquid may not be used, and it may not be possible to adequately increase the concentration of an image quality improvement treatment liquid so as to improve the image quality in printing. Therefore, the degree of freedom of treatment using the image quality improvement treatment liquid is limited, thereby making it difficult to use image quality improvement treatment liquid capable of remarkably preventing bleeding.
With the view of overcoming the above circumstances, Patent Document 4 proposes a technique in which the image quality improvement treatment liquid is applied to the printing surface of the recording medium (sheet) by using an application roller. According to this technique, it becomes possible to widen the range of the viscosity (i.e., increase the viscosity) of the image quality improvement treatment liquid and include various components capable of effectively improving the image quality with higher density in the image quality improvement treatment liquid.
On the other hand, it is also known that the less amount of image quality improvement treatment liquid applied to a medium such as a sheet becomes, the less the consumption amount of a fixing liquid in the image forming apparatus becomes. As a result, the cost in printing may be reduced and time for drying may be reduced so as to provide fast printing.    Patent Document 1: Japanese Patent Application Publication No. 2006-205465    Patent Document 2: Japanese Patent Application Publication No. 2001-301138    Patent Document 3: Japanese Patent Application Publication No. S64-9279    Patent Document 4: Japanese Patent Application Publication No. 2006-45522
However, there is a drawback. FIGS. 5A through 5C show a case where an image quality improvement treatment liquid 71 in a liquid state is simply applied to a surface of a medium (sheet) 72 such as high-quality paper by using an application roller 73. In this case, as described above, in order to reduce the amount of image quality improvement treatment liquid 71 to be applied to the surface of the medium (sheet) 72, it is necessary to reduce the thickness of the image quality improvement treatment liquid 71 formed on the surface of the application roller 73 (see FIG. 5A). Further, since the printing surface of the medium (sheet) 72 generally has convex sections and concave sections, the printing surface of the medium (sheet) 72 cannot be in full contact with the surface of the application roller 73 (see FIG. 5B). As a result, image quality improvement treatment liquid 71 having such a thin thickness cannot be fully applied to the printing surface of the medium (sheet) 72 (see FIG. 5C). More specifically, when it is assumed that 20 mg of the image quality improvement treatment liquid 71 is to be applied to the printing surface of an A4 sheet, it is necessary that the thickness of the image quality improvement treatment liquid 71 in a liquid state formed on the surface of the application roller 73 becomes about 0.32 μm (when transfer rate is assumed to be 100%). However, the height of the roughness of the printing surface of the high-quality paper is typically in a range from about 10 μm to about 20 μm. Therefore, in this case, it is not practically possible to uniformly apply the image quality improvement treatment liquid 71 in a liquid state to the surface of the medium (sheet) 72 by using the application roller 73. Further, when the image quality improvement treatment liquid 71 in a liquid state is applied to the printing surface of the medium (sheet) 72 using the application roller 73, in order to apply the image quality improvement treatment liquid 71 in a liquid state to the entire printing surface of the medium (sheet) 72, it may be difficult to reduce the amount of the image quality improvement treatment liquid 71 in a liquid state to be applied to the printing surface of the medium (sheet) 72 to about 100 mg or less per A4 sheet. As a result, the printed sheet may be curled or wrinkled; and the consumption amount of the image quality improvement treatment liquid 71 may be increased, thereby increasing the cost per printed sheet.